Synthesis, crystal structures, electronic structure and magnetic behaviourof the trithiatriazapentalenyl radical, C2S3N3

A novel synthesis of the title compound, C2S3N3 (1) is reported. X- and K-b and EPR spectra on dilute solutions of 1 indicate delocalisation of the unp aired spin density over both heterocyclic rings in agreement with DFT calcu lations. An XRPD study indicates that it crystallises in two morphologies w ith both phases formed during vacuum sublimation. The XRPD studies indicate that on cooling below 230 K, only the triclinic phase (P (1) over bar) bec omes detectable, whereas on warming above 320 K, just the monoclinic phase (P2(1)/c) becomes observed. The crystal structure of the monoclinic phase h as been examined by variable temperature single crystal X-ray diffraction i n the region 300-225 K and reveals a regular pi -stacked structure. A cryst al structure of the triclinic phase is reported at 150 K and exhibits a dim eric pi -stacked motif. Susceptibility measurements show that the monoclini c phase is paramagnetic whereas the triclinic phase is diamagnetic. This ra dical exhibits thermal hysteresis with a wide range of bistability; EPR and magnetic susceptibility measurements indicate Tc-down arrow = 234 K, and T c-up arrow = 317 K. The magnetic behaviour of the monoclinic phase is consi stent with strong antiferromagnetic exchange interactions between open shel l doublet states (J = -320 K) along the pi -stacking direction, although si gnificant inter-stack interactions are required to model the data adequatel y. In contrast the dimeric phase is essentially diamagnetic, with the resid ual paramagnetism indicating a very large singlet-triplet separation (|2J| > 2000 K). The magnetic exchange interactions in both phases are probed thr ough a series of DFT calculations using the broken-symmetry approach. These confirm the presence of strong magnetic exchange interactions along the pi -stacking direction in the high temperature phase (2J = -182 K), but with additional interstack interactions which are an order of magnitude smaller. Calculations on the triclinic phase indicate that it is best considered as a dimer with an open-shell singlet state with a very large singlet-triplet separation (2J = -2657 K). The magnitude of J for both phases from theory and experiment are in good agreement. The origin of the thermal hysteresis is attributed to the presence of two energetically similar structures which have a low energy barrier to interconversion. The thermodynamic parameters associated with the interconversion process have been probed by DSC studie s. It confirms the first order nature of the transition with Tc-down arrow = 232.3 K (DeltaH(down arrow) = 1.41 kJ mol(-1), DeltaS(down arrow) = 6.0 J mol(-1) K-1) and Tc-up arrow = 320.5 K (DeltaH(up arrow) = 1.86 kJ mol(-1) , DeltaS(up arrow) = 5.8 J mol(-1) K-1).